Wireless management system for energy storage systems

ABSTRACT

The wireless management system for energy storage systems includes a plurality of smart cells arranged into a two-dimensional array and a plurality of distributed management units. The smart cells and management units communicate with each other wirelessly via capacitive coupling (not radio). The communication links have extremely short range (typically under 3 mm), are relatively directional, and are electronically-steerable in the plane of the array. The smart cells may also include at least one power resistor and switch for passive cell balancing. The circuitry in the smart cells is typically incorporated into a flexible sheet that wraps around the energy storage device like a label. This system is extremely reliable because it is massively redundant, fault tolerant, and eliminates the wires used in conventional monitoring systems.

BACKGROUND

The invention relates generally to the field of electrical energystorage, and more specifically to the field of managing energy storagesystems to maximize performance and avoid damage resulting fromover-charging or over-discharging the cells.

Energy storage systems come in an enormous variety of sizes and shapes,but almost always include a plurality of energy storage devices.Batteries are the most common type of energy storage device by far, butsupercapacitors are gaining popularity because they offer a number ofadvantages over batteries, such as faster charging and higher peakdischarge currents. The energy storage devices typically come instandard form factors, and one of the most popular is the “18650”; acylindrical-shaped device, 18 mm in diameter, and 65 mm long. Forexample, an electric hand drill typically uses about four 18650 devices,while an electric car may use up to 10,000 cells.

FIG. 1 shows a first example of a conventional energy storage system 10that includes: a positive terminal 11 and a negative terminal 12 forconnecting the system 10 to a load and/or a charger; a plurality ofenergy storage devices 13; and optionally, a plurality of fuses 14designed to prevent a fire if one of the energy storage devices becomesshort-circuited by an internal fault. Sixteen of the energy storagedevices 13 are connected in parallel, thereby enabling the system 10 toproduce sixteen times the current of a single device. This is commonlyreferred to as a “16P” configuration, where “P” stands forparallel-connection. FIG. 2 shows a second example of a conventionalenergy storage system 20 wherein sixteen energy storage devices areconnected in series to produce sixteen times the voltage of a singledevice. This is commonly referred to as a “16S” configuration, where “S”stands for series-connection. Furthermore, FIG. 3 shows a third exampleof a conventional energy storage system 30 wherein four groups of energystorage devices are connected in series, and each group consists of fourdevices connected in parallel, producing four times the voltage andcurrent of a single device. This is commonly referred to as a “454P”configuration.

A key challenge when designing an energy storage system is assuringsafety and reliability. A key aspect of this challenge is assuring thatnone of the energy storage devices in the system are ever over-charged,or over-discharged. Therefore, every multi-device energy storage systemneeds the ability to monitor the State of Charge (SoC) of every energystorage device, and terminate the charging and discharging when the SoCfor any device exceeds predetermined limits. For example, an energystorage system that uses batteries almost always includes a BatteryManagement System (BMS) that is usually in the form of a circuit cardassembly.

FIG. 4 is a high level diagram of a 16S battery pack 40 with aconventional BMS 41. The BMS 41 includes a logic controller 42(typically a microcontroller with firmware) that determines when any ofthe energy storage devices 13 are fully charged (or discharged) and, inresponse, sends a stop signal to the charge controller and/or load (notshown) via a serial interface 43. The logic controller 42 may considermany factors in its decision to send the stop signal, but the dominantfactor is typically the voltage across each of the energy storagedevices 13. The logic controller 42 monitors the voltages via aplurality of wires 43, differential amplifiers 44, a multiplexer 45, andan analog-to-digital converter 46 with a precision voltage reference.When one of the voltages exceeds some predetermined threshold the BMStells the charge controller and/or load to terminatecharging/discharging.

Another important variable the logic controller 42 typically considersis temperature, because the performance of many energy storage devicetypes—notably lithium-ion batteries—is temperature-dependent. Therefore,a conventional BMS usually includes at least one temperature sensor 47.Ideally, each energy storage device 13 would have its own dedicatedtemperature sensor, but this is normally not practical due to the largenumber of required extra wires, so most conventional battery packdesigns have only a small number of temperature sensors—often justone—placed at strategic locations in the pack, where the temperature isexpected to approximate the average temperature for all the energystorage devices.

Additionally, the conventional BMS 41 typically includes circuitry forcharge balancing. Charge balancing is necessary because no two energystorage cells are exactly alike; rather they always have slightlydifferent energy storage capacities, which leads to imbalances. Forexample, when the battery pack 40 reaches the End of Discharge (EoD)state, some energy storage devices may still have as much as 15% energyremaining, while others may have as little as 5%. Unless the energystorage devices are periodically rebalanced to equal energy states, thismismatch increases with every charge/discharge cycle.

One charge balancing technique is called active balancing, but it is notoften used because it is relatively complex and expensive. Passivebalancing is the most common technique, and typically just requires aresistor 48 and electronically-controlled switch 49 for each energystorage device, as shown in FIG. 4.

Thus, the management process for the battery pack 40 typically includespassively balancing the energy storage devices while the system ischarging. The devices that have the most stored energy at the start ofthe charging process have their switches 49 closed, thereby allowingsome of the charging current to go around them via theparallel-connected resistor 48. This slows the charging rate of thesedevices, giving the other devices, i.e., the ones with the lower initialstate, time to catch up. Ideally, all the energy storage devices reach100% SoC nearly simultaneously, then the switches 49 are all opened andthe charging current is terminated.

The conventional BMS 41 has several well known disadvantages. First, itis inflexible. There is no one-size-fits-all BMS that can be used inevery battery pack so the designer must choose a BMS that exactlymatches the specific requirements for each particular pack design. Forexample, the balance resistors 48 and the switches 49 in a 4S8P packmust be rated for eight times the power dissipation they would need in a4S pack.

Second, the conventional BMS 41 is relatively expensive. The sense wires43 often must be soldered (and often glued) in place by hand, and touchlabor adds significant cost. Additionally, the circuit card assemblytypically requires extra components (e.g., diodes, fuses) to prevent afire in the case that one of the wires 43 breaks loose and touchesanother node that is at a significantly different voltage potential;these extra components can add significant cost.

Third, the conventional BMS 41 is relatively unreliable. One reason isthe lack of redundancy. For example, if one of the power resistors 48fails, then the energy storage devices can get dramatically out ofbalance and the whole system may fail. Additionally, when the wires 43are hand-soldered, the joints tend to be of relatively low quality andsometimes break.

Fourth, the conventional BMS 41 is relatively susceptible to common-modenoise. For example, in FIG. 4, if we assume each of the energy storagedevices 13 are charged to 3.5V, then the diffamp 44 at the top of thesixteen cell stack is (15)(3.5)=52.5V above the local ground (usuallythe negative terminal 12). The voltage across the top cell must bemeasured with about 1 mV accuracy, or better. So the requiredcommon-mode rejection ratio (CMRR) is very large,

20 log(52.5/0.001)=94.4 dB.  [Equation 1]

Fifth, the conventional BMS 41 does not get the best performance out ofa battery pack. The conventional BMS 41 usually only produces a roughestimate of State of Charge (SoC) because the data set (e.g., just thevoltage across each parallel-connected group of energy storage devices,and temperature at a few points) is so limited. This imprecision forcesdesigners to set the cutoff limits for charging/dischargingconservatively, thereby under-utilizing the battery packs full chargecapacity.

A wireless management system could potentially overcome all of theabove-mentioned disadvantages of the conventional BMS. In a wirelesssystem each energy storage device (or group of parallel-connected energystorage devices) includes its own monitoring circuit and typically itsown charge balancing circuit, and all the devices (or device groups)send their status information (e.g., voltage, temperature, etc.) to acentral management unit. The management unit analyzes all the data,tells the energy storage devices when to enable/disable their balancingcircuits, and communicates with the charge controller and/or load.

A wireless management system offers many advantages. First, itfacilitates more data collection which could lead to more accurate SoCestimates, thereby significantly increasing the usable capacity of theenergy storage system. Second, it may be more reliable because iteliminates all failure modes associated with broken wires, and addsmassive redundancy in the monitoring and balancing circuitry. Third, itis much more robust against common-mode noise because each monitoringcircuit has its own local ground reference. And fourth, it may reducecost by eliminating the significant amount of components and touch laborassociated with the sense wires.

But, despite all these advantages wireless management systems are stillrare because of significant technical and cost challenges. Obviously,the main technical challenge is how to implement the wirelesscommunication network, and there are many possible approaches thatdiffer in their choice of media (e.g., radio, infrared light, magneticfields, etc.) and network topology. The ‘topology’ of a communicationnetwork is the arrangement of the links that connect the nodes together.A node is any network-connected device such as a computer, a cell phone,a traffic light, or a cash register. A link is a communication pathbetween two or more nodes. FIG. 5 shows the seven basic networktopologies, where the circles represent the nodes and the linesrepresent the links. Every possible communication network is in the formof one of these topologies, or a combination of them.

FIG. 6A is a high-level diagram of a first example of a prior artwireless system 60 as disclosed in U.S. Pat. No. 8,917,566 to Lee, thecontents of which are herein incorporated by reference in theirentirety. The prior art wireless system 60 includes a plurality ofenergy storage devices 13, each device being connected to a SensorBattery Management Unit (S-BMU) 61. Each S-BMU includes a radiotransceiver with an antenna 62 and is in communication with a MasterBattery Management Unit (M-BMU) 63 via its own radio transceiver andantenna 64. Lee discloses that this network uses a star topology (FIG. 5diagram b) because each S-BMU only talks to the M-BMU; in other words,the S-BMUs do not talk directly to each other.

The system 60 appears simple at first but may not be very practical inreality because of significant electromagnetic challenges. Inparticular, Lee gave no details about the S-BMU antennas 62. FIG. 6B isa sketch of one possible implementation of the wireless system 60 toexplain why this omission is significant. The antennas 62 in FIG. 6B aresketched as small stubs just to make them clearly visible and to showthat the antennas must be on the ends of the energy storage devices 13to have a line-of-sight to the M-BMU antenna 64. But in a practicalsystem the S-BMU antenna 62 must have a low profile (i.e., a patchantenna) that essentially lays flat against the surface of the energystorage device so as not to get in the way of the bus bars. FIG. 6B doesnot show any bus bars for the sake of simplicity but, as previouslynoted, the bus bars could have many possible configurations, as shown inFIGS. 1-3. Naturally, one with ordinary skill in the art of electronicengineering would be left wondering how Lee avoided having the bus barsessentially cover the S-BMU antennas. But Lee provides no details, so itis unclear how the radio waves escape from the array of energy storagedevices so they can reach the M-BMU. Moreover, the energy storagedevices may not all have the same orientation. As was shown in FIGS.2-3, half the devices in many systems are oriented with their positiveend facing up, while the other half of the devices have their negativeend facing up. In other words, imagine FIG. 6B with half of the S-BMUantennas on the bottom side, facing away from the M-BMU, thereby makingthe problem of line-of-sight even more difficult. Additionally, therecould easily be more problems caused by multi-path reflections fromnearby metal surfaces, particularly if the system 60 is in a metalenclosure, as it likely would be when used in an electric vehicle. Insummary, a wireless system that uses radio for near-field communicationbetween hundreds or even thousands of network nodes can be fraught withchallenges and complications that must be dealt with on a case-by-casebasis.

FIG. 7A is a high-level diagram of a second example of a prior artwireless system as disclosed in U.S. Pat. No. 9,203,118 to Lenz, thecontents of which are herein incorporated by reference in theirentirety. Lenz disclosed a system 70 that includes a plurality ofstacked flat smart cells 71 (FIG. 7B) and a Battery System Manager (BSM)72. The smart cells 71 and the BSM 72 communicate wirelessly with eachother via capacitive coupling rather than radio. Each smart cell 71includes a flat energy storage device 13; a Monitor Unit (MU) 73; andtwo pairs of conductive plates 74 for communication via capacitivecoupling. The flat smart cells 71 have one pair of conductive plates 74on either side, allowing each cell 71 to talk to the cell directly aboveor below it in the stack.

The system 70 disclosed by Lenz solves the electromagnetic problemsassociated with the system 60 disclosed by Lee, but does so at the costof lower reliability. The Lee system 60 is inherently redundant becauseif a single S-BMU 61 fails, the rest of the system may continue tofunction normally. But the Lenz system 70 has no redundancy, so afailure in any MU 73 may cause the entire system to fail. This meansthat Lenz's system 70 is essentially limited to applications inrelatively small energy storage systems (e.g., cell phones and laptopcomputers) because the overall system failure rate is approximately thefailure rate of one cell 71 multiplied by the number of cells.Additionally, Lenz's system 70 has the serious disadvantage of beinglimited to flat energy storage devices, which are rarely used outside ofvery small energy storage systems.

There exists, therefore, an industry need for a wireless managementsystem that is well suited to application in medium and large energystorage systems.

SUMMARY

One embodiment for a wireless management system for energy storagesystems as disclosed herein includes: an energy storage system; a smartenergy storage cell used in the system; a flexible circuit assembly usedin some embodiments of the smart cell; an integrated circuit used in thesmart cell and flexible circuit assembly; and a management unit for atleast monitoring and controlling the smart cells. The energy storagesystem may include positive and negative terminals forcharging/discharging the system; a plurality of smart cells arrangedinto a two-dimensional array and coupled to the positive and negativeterminals; and at least one management unit in communication with atleast one of the smart cells. In one embodiment, the smart cell includesplus and minus terminals for connecting the smart cell to at least theenergy storage system; at least one energy storage cell; a plurality ofconductive plates surrounding the at least one energy storage device inthe two-dimensional plane of the array for communicating with othersmart cells and/or management units in the system via capacitivecoupling; and a cell controller circuit for at least producing datarelated to the state of the energy storage device(s) and communicatingvia the plurality of conductive plates. The flexible circuit assemblyincludes the plurality of conductive plates and the cell controllercircuit. The cell controller circuit may include a data acquisitioncircuit for producing the data related to the state of the energystorage devices; at least one driver for sending data via the conductiveplates; at least one receiver circuit for receiving data via theconductive plates; and a logic circuit for at least transferring databetween the data acquisition circuit, drivers, and receivers. The cellcontroller circuit may typically be included within the integratedcircuit. Furthermore, some embodiments of the system, smart cell, andflexible circuit assembly include at least one resistor andelectronically controlled switch for passive balancing of the energystorage devices. Said electronically controlled switch(es) may also beincluded within the integrated circuit.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings illustrate the invention. In such drawings:

FIG. 1 is a prior art example of an energy storage system comprisingsixteen energy storage devices connected in parallel;

FIG. 2 is another prior art example of an energy storage systemcomprising sixteen energy storage devices connected in series;

FIG. 3 is another prior art example of an energy storage systemcomprising four energy storage device groups in series;

FIG. 4 is another prior art example showing a battery pack with aconventional Battery Management System (BMS);

FIG. 5 is a set of high-level diagrams of the seven basic topologies ofcommunication networks;

FIG. 6A is a high-level block diagram of a prior art wireless managementsystem utilizing radio for communication;

FIG. 6B is a high-level diagram showing aspects of a physicalimplementation of the system shown in FIG. 6A;

FIG. 7A is a high-level block diagram of a prior art system thatutilizes capacitive coupling for communication;

FIG. 7B is a high-level sketch of the system shown in FIG. 7A;

FIG. 8A is a high-level block diagram of an energy storage system thatincludes the wireless management system disclosed herein;

FIG. 8B is a high-level sketch of one possible physical implementationof the system shown in FIG. 8A;

FIG. 9 is a simplified schematic diagram illustratingextremely-short-range wireless communication via capacitive coupling;

FIG. 10 is a waveform diagram showing an example encoding scheme;

FIG. 11A is a perspective view illustrating one embodiment of a smartcell having a plurality of conductive plates for communication viacapacitive coupling;

FIG. 11B is a top plan view illustrating two groups of smart cellspacked together in various ways to form an array;

FIG. 12A is a perspective view of another embodiment of a smart cellthat includes four energy storage devices and a plurality of conductiveplates that communicate via capacitive coupling;

FIG. 12B a top plan view of a group of four smart cells packed togetherin various ways to form a two-dimensional array;

FIG. 13A is a top view of some of the cells from FIG. 10B coupled to acommon-mode noise source;

FIG. 13B is a simplified equivalent circuit diagram showing theparasitic capacitances between two of the smart cells from FIG. 12A;

FIG. 14A is a top view of a smart cell with twelve conductive plates(A-L);

FIG. 14B is a table for selecting which plates to use for communicationin a desired direction;

FIG. 15 is a high-level diagram of the network topology;

FIGS. 16A and 16B are floor plan views of an energy storage systemhaving forty smart cells in a 5×8 array illustrating that thecommunication network can be reconfigured in many ways by setting the upand down directions for each smart cell;

FIG. 17 is a high-level block diagram of a smart cell showing details ofthe SCC;

FIG. 18 is a schematic diagram of an embodiment of the smart cell;

FIG. 19 is a perspective view of one embodiment of the smart cell havinga flexible circuit assembly;

FIG. 20 is a layout of one embodiment of the flexible circuit assembly;

FIG. 21 is a cross-sectional view of the flexible circuit assembly,taken about the line 21-21 in FIG. 20; and

FIG. 22 is a high-level block diagram of a management unit.

DETAILED DESCRIPTION

FIG. 8A is a high-level diagram of an energy storage system 80 thatincludes the wireless management system disclosed herein, and FIG. 8B isan illustrative example of how the system 80 may be constructed. Thesystem 80 includes a plurality of smart cells 81 arranged into atwo-dimensional array; and at least one Distributed Management Unit(DMU) 82 for managing the plurality of smart cells. Each smart cell 81includes at least one energy storage device 13; a Smart Cell Controller(SCC) 83; and a plurality of conductive plates 84 surrounding the energystorage device(s) 13 in the plane of the two-dimensional array. The SCC83 selects plates and utilizes them for communication with a DMU orother SCCs. The DMU 82 also includes a plurality of conductive plates85—similar to the plates 84 on the smart cells—for communicating with atleast one SCC 83. The DMU 82 is typically a small flat circuit cardassembly oriented with its plates 85 running parallel to the plates 84on the smart cells 81 to maximize the capacitance therebetween. The DMU82 may also include a network interface 86 for communicating with atleast another DMU in the system 80 via a management network 87. Thenetwork 87 may be wired or wireless; for example the network 87 could bea Universal Serial Bus (USB), a Controller Area Network (CAN bus), or alow-power radio network such as ZigBee, Bluetooth, etc. The system 80may also include a host computer 88 for providing a user interface, oradditional data processing power. If the network 87 is wired (such as inthe CAN bus example) then the host 88 may also provide power to each DMU82 via the network 87. The network 87 may also be linked to a chargecontroller and/or load 89.

Briefly, each of the smart cells 81 produces data related to its energystorage state, and transmits that data to one of the DMUs 82 via aplurality of cell-to-cell communication links. The communication linksutilize the conductive plates 84 for capacitive coupling, similar toLenz. But, unlike Lenz, the direction of communication can be steered inthe plane of the two-dimensional array by selecting which conductiveplates 84 each cell 81 utilizes.

One advantage of the system 80 over the prior art is its inherentscalability. FIGS. 8A-8B illustrate one example embodiment of arelatively small version of the system 80, i.e., sixteen smart cells 81arranged into a 4×4 rectangular array with two DMUs 82 as shown. But,the system 80 could easily be expanded to thousands of cells, and dozensof DMUs. Furthermore, the cells 81 may be electrically coupled by busbars (not shown) in any series and/or parallel combination, such asthose shown in FIGS. 1-3.

In contrast, the systems taught by Lee and Lenz are not inherentlyscalable. The Lee system 60 is difficult to scale because of its starnetwork topology. The network gets slower and slower as the number ofS-BMUs increases. The Lenz system 70 is difficult to scale because thefailure rate increases approximately linearly with the number of cells,i.e., scaling the Lenz system 70 results in a undesirably higher failurerates.

The system 80 disclosed herein overcomes the disadvantages of Lee andLenz. First, unlike Lee, the system 80 does not use a star networktopology, so it does not slow down significantly as it scales up.Second, unlike Lenz, the failure rate of the system 80 may actuallydecrease as the number of cells increases because the network topologyis reconfigurable. For example, if one of the smart cells 81 or DMUs 82fails, the network connections can be rerouted around the failed device.Notably, as the system 80 grows larger, it can tolerate a higher numberof failures because there are more alternative ways to reroute thenetwork connections. Finally, unlike Lenz, the system 80 may usecylindrical energy storage devices, rather than being limited to flatenergy storage devices.

FIG. 9 is a simplified schematic showing the concept ofextremely-short-range wireless communication via capacitive coupling inmore detail. A differential driver circuit 92 is connected to twoconductive plates 84 a, 84 b and a differential receiver circuit 93 isconnected to two more plates 84 c, 84 d. The differential driver 92 istypically just a complementary digital output driver used in manyintegrated circuits. When the first plate 84 a is driven high, the otherplate 84 b is driven low, and vise versa. The voltage difference betweenthe two plates 84 a, 84 b creates electric field flux lines 91 betweenand around them. For example, if the driver is powered by 3.5V, and thespacing between the two plates 84 a, 84 b is 1 mm, then the electricfield potential will be about 3.5V/mm at the midpoint between the twoplates 84 a, 84 b. The second pair of plates 84 c, 84 d is close to thetwo plates 84 a, 84 b, i.e., the distance D is typically about 1 mm.Thus, the pair of plates 84 c, 84 d are well within the electric field91 produced by the two plates 84 a, 84 b and consequently, there istypically at least 50 mV of potential therebetween. The differentialreceiver circuit 93 amplifies this small potential by a factor of atleast a few thousand. The receiver circuit 93 is typically just anordinary voltage comparator, with a high-impedance dc bias networkcoupled to its inputs, and a few millivolts of hysteresis to rejectnoise. Note that if the distance D increases to more than about 3 mm,the potential between the plates 84 c, 84 d may be too small to overcomethe hysteresis and the output of the receiver 93 will be flat.

Capacitive coupling could be achieved with just one conductive plate 84in each smart cell 81 because, in an energy storage system, the smartcells are electrically coupled by bus bars, and the bus bars provide areturn path for the displacement current flowing through the plates. Oneadvantage of differential signaling, with two plates per smart cell asillustrated in FIG. 9, is common-mode rejection. Although, whenincorporated into the embodiments disclosed herein, differentialsignaling also has the ability to determine the relative orientations oftwo adjacent smart cells; and improved angular resolution in thedirection control, as described in detail below in relation to FIGS.13A-14B.

Orientation refers to which end of the cell (plus or minus) is facingup. For example, in FIG. 1, all the cells are oriented with their plussides up, while in FIGS. 2-3 half of the cells are flipped, i.e., withtheir plus sides down. When two adjacent smart cells have the sameorientation, as in FIG. 9, the data is passed between them verbatim.But, when one of the cells is flipped relative to the other, the datastream coming out of the receiver 93 is inverted compared to the datastream going into the driver 92.

These data inversions may be detected with coding techniques, andadvantageously utilized when mapping the array. FIG. 10 shows twowaveforms for the ‘P’ and ‘N’ signals on the plates 84 a, 84 b in FIG.9, respectively. Every message between two smart cells starts with async pattern before the data. In this example the data is just threearbitrary bits ‘011’. Whenever a digital bit stream is ac-coupled,Manchester encoding is typically used, wherein each bit interval isdivided into two equal subintervals that contain complementary voltagelevels. For example, a ‘1’ symbol is encoded as a high level for thefirst subinterval, then a low level for the second subinterval.Accordingly, a ‘0’ symbol is the opposite: low for the firstsubinterval, then high for the second. Manchester encoding always keepsthe average voltage (the so-called baseline) relatively near the middlebetween the high and low levels, even when the data stream contains longstretches of consecutive identical digits (e.g., 0000000 or 1111111).This minimizes so-called baseline wandering, thereby reducing a sourceof bit errors known as pattern-dependent jitter. But Manchester encodingoffers other advantages, including the ability to differentiate controlsymbols from data symbols. For example, in FIG. 10 the sync pattern ishigh for three subintervals, then low for three subintervals, then highfor one subinterval. This pattern distinctly stands out as a controlsymbol because there is no way to produce it with any sequence of ‘1’and ‘0’ symbols. Therefore, if a smart cell receives an inverted syncpattern, it knows the cell that sent it has the opposite orientation,and in response the receiving cell will invert all subsequent data itreceives from that flipped cell.

FIG. 11A shows a first embodiment of the smart cell 81 constructed witha single cylindrical-shaped energy storage device 13. For suchcylindrical-shaped cells, twelve conductive plates 84 may be used,distributed at 30° intervals surrounding the energy storage device 13.The plates are sandwiched between two electrically-insulating layers:the inner insulating layer 100 preventing electrical contact between theplates 84 and the conductive metal case of the energy storage device 13;and the outer insulating layer 98 covering the plates 84 to preventelectrical contact with the plates on other nearby smart cells.

FIG. 11B shows why at least twelve plates are typically used forcylindrical-shaped smart cells. There are two ways of packingcylindrical cells together: rectangular, and hexagonal. The first array101 is a top view of nine smart cells 81 a-81 i packed in a 3×3rectangular array; and the second array 102 is a top view of seven smartcells 81 j-81 p packed in a hexagonal array. The cell 81 e at the centerof the first array 101 can communicate with only four of its neighbors(81 b, 81 d, 81 f, and 81 h) because the others (81 a, 81 c, 81 g, and81 i) are too far away. But cell 81 m, at the center of the second array102, can communicate with all six of its neighbors (81 j, 81 k, 811, 81n, 81 o, and 81 p) Note that the cell 81 m has neighbors at 60°intervals around its perimeter, while the cell 81 e has reachableneighbors at 90° intervals around its perimeter. The greatest commondenominator of 90° and 60° is 30°; therefore, at a minimum, eachcylindrical-shaped smart cell as shown in FIG. 11A should have enoughplates to allow it to steer its communication links in 30° increments orless. This entails a minimum of twelve plates.

Nevertheless, smart cells that are not cylindrical-shaped can use a lotfewer plates. For example, FIG. 12A shows another embodiment of thesmart cell 81 including four energy storage devices 13, giving it across-section that is approximately square with rounded corners. FIG.12B is a top view of four such smart cells packed in a 2×2 array,showing that they always have a maximum of four reachable neighbors at90° intervals. So a square smart cell as shown in FIG. 12A requires onlyenough plates to communicate in four directions.

One of the technical challenges when working with cylindrical-shapedsmart cells arises from the fact they can be arbitrarily rotated. FIG.13A shows a subset of the smart cells from FIG. 11B in more detail,where each smart cell has twelve plates 84 (labeled ‘A’ through ‘L’) andis rotated randomly. Sometimes two smart cells happen to alignfortuitously; for example, plates B/C on cell 81 m happen to line upwell with plates E/F on cell 81 k. But often pairs of cells will bemisaligned; for example, plates C/D on cell 81 l do not line up wellwith plates J/K on the cell 81 m. To explain why this is a problem, acommon-mode ac voltage source 121 is connected between the local ground122 for the cell 81 m, and another node 123 that is the local ground forthe other three cells. This produces an electric field with flux lines124 approximately as shown.

The affects of plate misalignment are made more clear by FIG. 13B whichis a simplified equivalent circuit diagram of the communication linkbetween the smart cells 81 l and 81 m in FIG. 13A. Note that the dashedwires are the traditional way of depicting parasitic components incircuit diagrams. There are six fixed, and four variable parasiticcapacitors in FIG. 13B. The fixed caps are determined by theconstruction of the cell; for example, Cu is the capacitance betweenplate J and the local ground 122 which is determined by the size ofplate J, the thickness of the insulating layer 100 between the plate andmetal casing of the energy storage device 13, and the dielectricconstant of the insulating layer. The four variable caps are functionsof the distance between the cells and the rotational alignment of thecells. First, if cell 81 l is not transmitting and the common-mode noisesource 121 is zero, then the resulting differential voltage betweenplates K/J is zero and the output of the receiver 93 is stationary(either stuck high or stuck low). Next, the noise source 121 is turnedon. If plates C/D are perfectly aligned with plates K/J, then thecircuit is balanced (C_(CK)=C_(DJ) and C_(CJ)=C_(KD)) and the voltagebetween plates K/J remains at zero. However, if the plates aremisaligned as shown in FIG. 13A, then the circuit is severely unbalanced(C_(CK)≠C_(DJ) and C_(CJ)≠C_(KD)) resulting in a differential voltagebetween plates K/J. If this voltage difference is greater than thehysteresis of the receiver 93, then bit errors result. Unfortunately,the hysteresis of the receiver 93 must be kept relatively small(typically on the order of 10 mV) because CKK and CJJ are typically atleast ten times larger than C_(CK) and C_(DJ) so the differentialvoltage output by the driver 92 produces a much smaller differentialvoltage at the receiver 93 because of voltage division.

FIG. 13A also shows the solution to this problem. Cell 81 n ismisaligned with cell 81 m, and yet the parasitic capacitances are stillbalanced because the flux lines are symmetrical. Cell 81 n achieves thisbalance by using plates K/A instead of K/L or L/A. This technique isshown more fully in FIGS. 14A-14B. FIG. 14A defines the direction angleθ starting from plate A and going clockwise in 15° incrementscorresponding to direction codes 0 to 23 and FIG. 14B shows which platesare used for each direction code. For example, if a cell wants totransmit or receive in direction 3 (45° clockwise from plate A) then itwould use plate B for positive (P) and plate C for negative (N). Notethat P and N correspond to the plates in FIG. 9. In other words, thisvariation of differential signaling provides 15° angular resolution withjust twelve plates.

FIG. 15 is a high-level diagram of the network topology 140. The smartcells 81 are organized into sub-networks called chains. A chain has aline topology (FIG. 5, diagram f) with a DMU 82 at the top. The smartcell at the bottom of each chain (bold circles) are called terminationcells, and all the others are called link cells. All the smart cells arephysically identical, but they have two modes of operation (link andtermination) that behave differently. Essentially, the DMU sets aparticular mode bit in the bottom cell, telling the cell that it's thefinal cell in the chain, and thereby changing its behavior. The DMUs arein communication with each other via a management network 87 thattypically has a bus topology (FIG. 5 diagram d). A computer 88 may alsobe connected to the bus 87 to at least provide a user interface.

Each link cell is given two direction codes which it uses to select theapplicable plates in accordance with the table in FIG. 14B. The firstdirection code points ‘up’ the chain toward the DMU, and the seconddirection code points ‘down’ the chain toward the termination cell. Notethat ‘up’ and ‘down’ are subjective terms (conventions) regarding dataflow, and do not represent literal spacial directions. A terminationcell is only given an ‘up’ direction code, because it is at the ‘bottom’of the chain.

FIGS. 16A-B both show floor plan views of the same energy storage system150 comprising a 5×8 rectangular array of cylindrical smart cells 81,and two DMUs 82 on either end. The lines represent the capacitivecommunication links. In both diagrams, four chains meander through thearray, but they follow different paths simply because the cells havebeen programmed with different ‘up’ and ‘down’ direction codes.

The DMUs assign the up and down directions for each cell, and canreassign them at will. In other words, the chains may initially be builtas shown in FIG. 16A, and then be rebuilt later as shown in FIG. 16B,just by changing the up and down direction codes in each smart cell.Furthermore, the DMUs cooperate with each other via the management bus87, so that none of the chains overlap or cross over each other, and nosmart cells are left out.

FIG. 17 is a high level block diagram of a smart cell 81 that includes:a positive terminal 160 and a negative terminal 161 for connecting thesmart cell 81 to an energy storage system; at least one energy storagedevice 13 coupled to the terminals 160-161; a plurality of conductiveplates 84 for communication via capacitive coupling; and an SCC 82. TheSCC 82 includes data acquisition circuitry for producing data related tothe state of the energy storage device(s). The data acquisitioncircuitry typically includes a differential amplifier 162, a multiplexer163, and an analog-to-digital converter 164 with a voltage reference165; at least one driver 92; at least one receiver 93; and a logiccircuit 166. The logic circuit is for at least transferring data betweenthe data acquisition circuit and the driver(s) and receivers(s), andselects which of conductive plates 84 are utilized for communication.The SCC 82 also typically includes a temperature sensor 167. The smartcell 81 also may include at least one resistor 169 and anelectronically-controlled switch 170 for passive charge balancing.Finally, the SCC 82 may include a switch matrix 171 that couples theconductive plates 84 to the driver(s) 92 and the receiver(s) 93. Theswitch matrix 171 allows for a relatively small number of drivers andreceivers by coupling them to the particular plates used for the givenup or down direction code. Alternatively, the switch matrix 171 could beomitted if there are enough drivers and receivers to hard-wire one ofeach for each direction code, in which case the drivers would need tohave tri-state outputs. Note that, in the context of this document, theschematic symbol for a battery is used to generally represent any typeof electrical energy storage device. For example, the energy storagedevice 13 could be a battery, a capacitor, or any combination ofbatteries and capacitors.

The smart cell may also include at least one additional sensor 168 forproviding more information about the state, or health of the energystorage cell(s). For example, some energy storage cells build upinternal pressure when they are damaged by over-charging orover-discharging, so the additional sensor could be a pressuretransducer or a strain gage.

The diffamp 162 scales the voltage of a cell so that it can be read bythe ADC 164. For example, a lithium-ion battery produces up to 4.4V atpeak charge, and as low as 2.8V at end-of-discharge. All the subcircuits(92, 93, 162-167) in the SCC must be able to operate over this supplyvoltage range, so the voltage reference 165 is typically about 2.5V.Obviously, 4.4V is higher than the reference, so the diffamp 162 mustreduce it, typically by a factor of two. This scaling function could beperformed by a simple resistor divider network instead of a diffamp, butthe diffamp takes up less space than a resistor divider when part of anintegrated circuit.

The ADC 164 and voltage reference 165 require relatively high precisionto estimate the SoC because the voltage of a typical lithium-ion batterychanges little (typically <100 mV) as it discharges from about 80%charge down to about 20% SoC. So the ADC 164 is typically a delta-sigmatype with at least 12-bits resolution. Furthermore, the voltagereference 165 requires relatively low long-term drift.

The logic circuit 166 includes registers or volatile memory for storingconfiguration settings received from a DMU. The configuration settingsinclude at least the up and down direction codes and at least one modebit for designating the smart cell as either a link or termination cell.The logic circuit 166 also includes at least one status register orvolatile memory that can be read by a DMU. At least one bit in thestatus register(s) represents the relative orientation of a neighboringsmart cell. For example, this status bit typically indicates if the nextcell down the chain has the same or opposite orientation relative to thecell that contains the status bit.

The logic circuit 166 may also include nonvolatile memory for storinginformation such as: a manufacturer's identification code, a serialnumber or random number, a date code indicating when the smart cell wasmanufactured, and/or calibration data related to the data acquisitioncircuitry. The logic circuit 166 may also include at least one bit forsecurity which, when asserted, inhibits reading at least some of thedata in the nonvolatile memory. For example, such security is typicallyimplemented as a polysilicon fuse, and blowing the fuse asserts the bit.

The logic circuit 166 may also include a first timer circuit forresetting at least some of the volatile memory to a predefined defaultstate in response to the smart cell not receiving any communicationsfrom a DMU or another smart cell within a predetermined time limit. Andthe logic circuit may also include a second timer for automaticallyturning off the balance switch(es) 170 after a predetermined time limitbeginning when the balance switch(es) were turned on.

An important characteristic of the SCC 82 is its low quiescent powerdissipation. For example, a typical lithium ion battery has aself-discharge rate of about 2% to 3% per month, and a well designed SCCshould not significantly add to that self-discharge rate. Therefore, theSCC typically has a sleep state wherein many of its internal circuits(e.g., the ADC 164, the voltage reference 165, and the drivers 92) areturned off, but some circuits (e.g., the logic 166, the switch matrix171, and at least one receiver 93) remain active so that the SCC 82 canwake up in response to a communication from another SCC or a DMU.

FIG. 18 is a schematic diagram of an embodiment of the smart cell 81wherein the SCC 82 is in the form of a monolithic integrated circuit 172that comprises the data acquisition circuitry (162-165), the driver(s)92, the receiver(s) 93, the logic 166, and typically the switch matrix171. Furthermore, FIG. 18 shows how the heat produced while balancing isspread out by using a plurality of resistors 169 a connected in parallelrather than a single resistor like in FIG. 17.

FIG. 18 also shows how two or more switches can be connected in seriesto improve reliability. The balance switches 170 a-170 b are typically aparticular type of transistor called a Metal-Oxide-SemiconductorField-Effect Transistor (MOSFET). If a single MOSFET is used and itfails by developing an internal short circuit—a common failure mode forMOSFETs—then the balance resistors 169 a would constantly drain theenergy storage device 13. Having two MOSFETs in series dramaticallylowers the probability of this type of failure because both 170 a and170 b would have to be shorted before inadvertently draining the energystorage device 13. It is also possible that one of the MOSFETs couldfail as an open circuit, thereby preventing the resistors 169 a fromever being used again, but this is much less concerning because smartcells are typically used in parallel-connected groups, so all the cellscan still balance as long as at least one smart cell in the group canclose its balance switches.

The switches 170 a-170 b could be part of the integrated circuit 172 butsometimes they are separate discrete devices, as shown in FIG. 18, sothat their power dissipation does not heat the integrated circuit 172which would affect the readings from the temperature sensor 167.

FIG. 19 is an embodiment of the smart cell 81 including a singlecylindrical-shaped energy storage device. A protective outer layer 98,typically a heat-shrink sleeve, is partially cut away to reveal aflexible circuit assembly 180 wrapped around the cell. The flexiblecircuit assembly 180 typically includes a tab 174 that folds over thetop of the cell for making electrical contact with the positive terminal160. Additionally, a plastic ring 175 typically protects the tab 174.

FIG. 20 is a typical layout of the outer surface of the flexible circuitassembly 180 laid out flat to further illustrate its features. Theoverall width of the assembly 180 is approximately equal to thecircumference of the energy storage device it wraps around. A relativelysmall conductive pad 160 a near the end of the tab 174 is for makingelectrical connection with the positive terminal 160 of the energystorage device. Another conductive pad 161 a is for making electricalconnection with the case of the energy storage device, which is also thenegative terminal 161. The conductive plates 84 for capacitivecommunication—in this example, twelve plates labeled A-L—are distributedevenly. A plurality of resistor sections 169 a may be coupled inparallel to form the resistor 169 from FIG. 17. The resistor sectionsare typically distributed evenly to spread out their heat dissipation.

FIG. 21 is a cross-sectional view of the assembly 180, taken about theline 21-21 in FIG. 20. A flexible substrate 190—typically made ofpolyimide or polyester, and about 12 μm thick—has copper traces 191about 6 μm thick etched on at least one side for making electricalconnections to the integrated circuit 172. The integrated circuit 172 issoldered to the copper traces and then covered by anelectrically-insulating layer 100 which is typically about 100 μm thick.The insulating layer 100 typically encapsulates the integrated circuit172 to relieve mechanical stresses on the solder joints as the assembly180 flexes. The conductive plates 84 and the resistors 169 a arenormally added to the outer side of the flexible substrate. Theresistors are typically made from a carbon paste or ink 195 applied bysilk-screening, ink jet printing, or spraying. This material istypically applied with a thickness of 15 μm or less, resulting in asheet resistance of approximately 5 to 10 Ohms per square. Theconductive plates can be made from the same material as the resistorsbecause the plates do not need to be highly conductive. The plates andresistors make electrical connection with the copper traces throughsmall windows 192 in the flexible substrate 190.

The manufacturing process for a smart cell 81 is typically nearly thesame as the process for manufacturing the energy storage device, butwith at least two additional steps. First, the flexible circuit assembly180 is wrapped around the energy storage device. The inner surface ofthe flexible circuit assembly may be coated with an adhesive to make itstick to the energy storage device like a conventional label. Second,the pads 160 a and 161 a are typically spot-welded to the terminals 160and 161 of the energy storage device just before the final step ofapplying the outer insulating layer 98.

The pads 160 a and 161 a are usually designed to facilitatespot-welding. The thickness of the pads is normally about equal to theoverall thickness of the flexible circuit assembly, and the pads areexposed on both sides. For example, there are typically windows 196 inthe flexible substrate 190 to expose the pads on the outer surface. Thepads are normally plated with the same material (usually nickel) as theplating on the energy storage device(s) terminals 160 and 161.

FIG. 22 is a high-level block diagram of the DMU 82. The DMU mayinclude: a plurality of conductive plates 85 for communicating with atleast one of the smart cells via capacitive coupling; at least onedriver circuit 92 for sending at least commands to at least one smartcell via the conductive plates; a least one receiver circuit 93 forreceiving at least data from at least one smart cell via conductiveplates; a network interface 86 for at least communicating with othermanagement units within the same energy storage system; and a logiccircuit 200 for at least conveying information between the networkinterface, the driver circuit(s), and the receiver circuit(s).

The logic circuit 200 is typically a microcontroller with memory, or aprogrammable logic device such as a FPGA or CPLD. The logic circuit 200may include a nonvolatile memory for storing information such as: amanufacturer's identification code; a serial number or random number; adate code indicating when the DMU was manufactured; and a floor plan ormap for building the chains. The logic circuit 200 may also include atleast one bit for security which, when asserted, inhibits reading atleast some of the data in the nonvolatile memory. For example, suchsecurity is may be implemented as a polysilicon fuse, and blowing thefuse asserts the bit.

The DMU 82 may require at least one supply voltage to operate at leastthe logic 200, the driver(s) 92 and the receiver(s) 93. The DMU may beadapted to directly utilize a supply voltage provided by the networkinterlace 86. For example, the network interface could be USB whichprovides +5 Vdc, and the DMU may run directly off this supply withoutthe need for any voltage conversion or regulation. Alternatively, theDMU may include a power supply circuit 201 for at least regulating thesupply voltage(s). Furthermore, the power supply circuit 201 may beadapted to receive power from the network interface 86, or from thepositive terminal 11 and the negative terminal 12 of the system 80.

1. An energy storage system, comprising: a positive terminal and anegative terminal for charging and discharging the energy storagesystem; a plurality of smart cells arranged into a two-dimensional arrayand coupled to the positive and negative terminals, each smart cellcomprising: at least one energy storage device, a plurality ofconductive plates surrounding the at least one energy storage device inthe plane of the two-dimensional array for communicating with multipleother smart cells within the array via a capacitive coupling, and a cellcontroller circuit producing data related to the state of the at leastone energy storage device and operable to receive a direction code and,in response to receiving the direction code, selecting at least oneplate from the plurality of conductive plates and utilizing the at leastone selected plate to communicate the data; and at least one managementunit for sending the direction code to the plurality of smart cells andreceiving the data from the smart cells via said capacitive coupling. 2.The energy storage system of claim 1, including a management network,wherein each management unit includes a network interface coupled to themanagement network for communicating with at least one other managementunit within the energy storage system.
 3. The energy storage system ofclaim 1, wherein the at least one of the management unit is incommunication with a device for charging the energy storage system. 4.The energy storage system of claim 1, wherein each smart cell furtherincludes at least one resistor coupled to the at least one energystorage device via at least one electronically controlled switch forpassive balancing of the at least one energy storage device.
 5. Theenergy storage system of claim 1, wherein the data produced by the cellcontroller circuit is selected from the group consisting of a voltageacross at least one energy storage device, a temperature of at least oneenergy storage device, and an internal pressure of the least one energystorage device.
 6. A smart cell for storing and providing energy in anenergy storage system, comprising: a positive terminal and a negativeterminal for connecting the smart cell to at least the energy storagesystem; at least one energy storage device coupled to the positiveterminal and the negative terminal; a plurality of conductive platessurrounding the at least one energy storage device for communicatingwith at least one other smart cell in the energy storage system via acapacitive coupling; and a cell controller circuit for drawing powerfrom the positive terminal and the negative terminal and producing datarelated to the state of the at least one energy storage device, the cellcontroller operable to receive a direction code and, in response toreceiving the direction code, selecting at least one plate from theplurality of conductive plates and utilizing the at least one selectedplate to communicate the data.
 7. The smart cell of claim 6, wherein thecell controller circuit is operable to select a pair of the plurality ofconductive plates for differential signaling.
 8. The smart cell of claim6, further including at least one resistor coupled to the at least oneenergy storage device via at least one electronically controlled switchfor passive balancing of the at least one energy storage device.
 9. Aflexible circuit assembly for a smart cell that includes at least oneenergy storage device, the flexible circuit assembly comprising: aflexible substrate adapted to wrap around the at least one energystorage device; a plurality of conductive plates distributed across theflexible substrate for communication via a capacitive coupling; apositive node and a negative node for coupling to the at least oneenergy storage device; and a cell controller circuit for drawing powerfrom the positive node and the negative node and producing data relatedto the state of the at least one energy storage device, the cellcontroller operable to receive a direction code and, in response toreceiving the direction code, selecting at least one plate from theplurality of conductive plates and utilizing the at least one selectedplate to communicate the data.
 10. The flexible circuit assembly ofclaim 9, including at least one resistor and at least one electronicallycontrolled switch coupled to the positive and negative nodes for passivecell balancing.
 11. An integrated circuit for controlling a smart cellhaving at least one energy storage device and a plurality of conductiveplates for communication with multiple other smart cells within anenergy storage system via a capacitive coupling, the integrated circuitcomprising: a data acquisition circuit for producing data related to thestate of the at least one energy storage device; a switch matrix forselecting at least one plate from the plurality of conductive plates; atleast one driver circuit for sending data via the at least one selectedplate; at least one receiver circuit for receiving data via the at leastone selected plate; and a logic circuit for controlling the switchmatrix and transferring data among the data acquisition circuit, the atleast one driver circuit, and the at least one receiver circuit.
 12. Theintegrated circuit of claim 11, wherein the logic circuit furtherincludes at least one configuration register for storing at least onedirection code that determines which of the plurality of conductiveplates are selected by the switch matrix.
 13. The integrated circuit ofclaim 12, wherein the selected at least one plate comprises at least apair of conductive plates suitable for differential signaling.
 14. Theintegrated circuit of claim 11, wherein the logic circuit furtherincludes at least one status bit with a state determined by a polarityof a received differential signal.
 15. The integrated circuit of claim11, further including at least one electronically-controlled switch forcontrolling a current flow through at least one resistor for passivebalancing of the least one energy storage device.
 16. The integratedcircuit of claim 11, further including a nonvolatile memory for storinginformation selected from the group consisting of a manufactureridentification number, a serial number, a date code, and a calibrationdata related to the data acquisition circuit.
 17. The integrated circuitof claim 12, wherein the logic circuit further includes at least onetimer for resetting the at least one configuration register to apredefined default state in response to an absence of any received datafor a predefined time interval.
 18. The integrated circuit of claim 15,wherein the logic circuit further includes at least one timer forautomatically turning off the at least one electronically-controlledswitch after a predefined time interval.
 19. A management unit formonitoring and controlling a plurality of smart cells in an energystorage system, comprising: a plurality of conductive plates forcommunicating with at least one of the plurality of smart cells via acapacitive coupling; at least one driver circuit for sending commands toat least one of the plurality of smart cells via the conductive plates;a least one receiver circuit for receiving data from at least one of theplurality of smart cells via the conductive plates; a network interfacefor communicating with other managements unit within the energy storagesystem; and a logic circuit for conveying information among the networkinterface, the at least one driver circuit, and the at least onereceiver circuit.
 20. The management unit of claim 19, including a powersupply circuit for providing power to the at least one driver circuit,the at least one receiver circuit, and the logic circuit, the powersupply adapted to receive power from at least one source selected fromthe group consisting of the network interface and the energy storagesystem.